Terahertz detector comprised of P-N junction diode

ABSTRACT

A method of forming a semiconductor detector including: forming a p-n junction diode in an active device layer of a silicon-on-insulator (SOI) substrate, the active device layer being formed on an insulator layer of the SOI substrate; forming a first opening through the insulator layer to access a backside of a first doped region of the diode, the first doped region underlying a second doped region of the diode; forming a back contact on a back surface of the first doped region and electrically connecting with the first doped region; forming a conductive interconnect layer on an upper surface of the SOI substrate, the interconnect layer including a first top contact providing electrical connection with the second doped region; and forming an electrode in the first opening on the backside of the detector structure, the electrode providing electrical connection with the back contact of the diode.

BACKGROUND OF THE INVENTION

The present invention relates generally to the electrical, electronicand computer arts, and, more particularly, to high-frequency detectors.

It has been demonstrated that high-frequency (e.g., terahertz (THz))detectors based on p-n junction diodes provide certain benefits overdiode-connected metal-oxide-semiconductor (MOS) transistors andheterojunction bipolar transistors (HBTs) due to their higher cornerfrequency, f_(c). Flicker noise, also referred to as 1/f noise, is oftencharacterized by the corner frequency between the region dominated bythe low-frequency flicker noise and the higher-frequency “flat-band”noise. MOS field-effect transistors (MOSFETs) typically exhibit a higherflicker noise and therefore a lower f_(c) for a given flat-band noise,compared to junction field-effect transistors (JFETs) or bipolartransistors. The higher flicker noise in MOSFETs is often attributed tothe presence of interface states between the gate dielectric and thechannel of the transistor.

A higher f_(c) allows the detector to achieve higher signal-to-noiseratio (SNR), which in turn provides a higher operation frequency.However, an upper limit of the operation frequency is still dictated, toa large extent, by an effective path length for carrier diffusion and aneffective series resistance of the diode.

BRIEF SUMMARY

Principles of the invention, in accordance with one or more embodimentsthereof, provide a novel diode structure that substantially reduces aneffective path length for carrier diffusion, and substantially reducesan effective series resistance of the diode structure, thus providing adetector that beneficially achieves a higher operation frequencycompared to conventional detectors.

In one aspect, an embodiment of the invention includes a method offabricating a semiconductor detector including: forming a p-n junctiondiode in an active device layer of a silicon-on-insulator (SOI)substrate, the active device layer being formed on an upper surface ofan insulator layer of the SOI substrate, the insulator layer beingformed on an upper surface of a carrier substrate of the SOI substrate;removing the carrier substrate to thereby expose the insulator layer;forming a first opening through the insulator layer to access a backsideof a first doped region of the p-n junction diode, the first dopedregion underlying a second doped region of the p-n junction diode, thefirst doped region having a first conductivity type associated therewithand the second doped region having a second conductivity type associatedtherewith which is opposite in polarity to the first conductivity type;forming a back contact on a back surface of the first doped region andelectrically connecting with the first doped region; forming aconductive interconnect layer on an upper surface of the SOI substrate,the interconnect layer comprising a first top contact providingelectrical connection with the second doped region; and forming anelectrode of the p-n junction diode in the first opening on the backsideof the detector structure, the electrode providing electrical connectionwith the back contact of the p-n junction diode.

In another aspect, an embodiment of the invention includes asemiconductor device comprising at least one p-n junction diode formedon a silicon-on-insulator (SOI) substrate, the p-n junction diodeincluding first and second doped regions formed in an active devicelayer of the SOI substrate, the first region being formed on an uppersurface of the second doped region and having a first conductivity typeassociated therewith, the second doped region having a secondconductivity type associated therewith, the second conductivity typebeing opposite in polarity to the first conductivity type. A depletionregion is formed at an interface between the first and second dopedregions. The p-n junction diode further includes a back contact formedon a back surface of the second doped region and electrically connectingwith the second doped region. A conductive interconnect layer is formedon an upper surface of the SOI substrate, the interconnect layerincluding a first top contact providing electrical connection with thefirst doped region. An electrode of the p-n junction diode is formed onthe backside of the semiconductor device, the electrode providingelectrical connection with the back contact.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

Techniques of the present invention can provide substantial beneficialtechnical effects. By way of example only and without limitation, one ormore embodiments may provide one or more of the following advantages,among other benefits:

-   -   substantially lower effective diffusion capacitance;    -   lower effective series resistance;    -   substantially higher signal-to-noise ratio;    -   mechanical flexibility.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following drawings are presented by way of example only and withoutlimitation, wherein like reference numerals (when used) indicatecorresponding elements throughout the several views, and wherein:

FIG. 1A depicts top plan and cross-sectional views of at least a portionof an exemplary terahertz (THz) detector structure based on a p-njunction diode fabricated using a complementarymetal-oxide-semiconductor (CMOS) process;

FIG. 1B is a circuit diagram depicting at least a portion of anexemplary detector circuit which uses a p-n junction diode which may beformed in a manner consistent with the p-n junction diode shown in FIG.1A;

FIGS. 2-10 are cross-sectional views depicting various stages of anillustrative semiconductor fabrication process in the formation of anexemplary THz detector structure, according to an embodiment of thepresent invention;

FIGS. 11-18 are cross-sectional views depicting various stages of anillustrative semiconductor fabrication process in the formation of anexemplary THz detector structure, according to another embodiment of thepresent invention;

FIG. 19 is a conceptual view depicting an illustrative waveform of anexemplary incidental signal which may be received by the exemplarydetector circuit shown in FIG. 1B; and

FIG. 20 is a schematic diagram depicting at least a portion of anexemplary array-based detection circuit in which aspects of the presentinvention may be employed, according to an embodiment of the presentinvention.

It is to be appreciated that elements in the figures are illustrated forsimplicity and clarity. Common but well-understood elements that may beuseful or necessary in a commercially feasible embodiment may not beshown in order to facilitate a less hindered view of the illustratedembodiments.

DETAILED DESCRIPTION

Principles of the present invention will be described herein in thecontext of illustrative detector devices comprised of p-n junctiondiodes and applications utilizing such detector devices. It is to beappreciated, however, that the specific structures, circuits and/ormethods illustratively shown and described herein are to be consideredexemplary as opposed to limiting. Moreover, it will become apparent tothose skilled in the art given the teachings herein that numerousmodifications can be made to the embodiments shown that are within thescope of the appended claims. That is, no limitations with respect tothe embodiments shown and described herein are intended or should beinferred.

Although the overall fabrication method and the structures for thedisclosed embodiments are entirely novel, certain individual processingsteps required to implement the device may utilize conventionalsemiconductor fabrication techniques and conventional semiconductorfabrication tooling. These techniques and tooling will already befamiliar to one having ordinary skill in the relevant art given theteachings herein. Moreover, many of the processing steps and toolingused to fabricate semiconductor devices are also described in a numberof readily available publications, including, for example: James D.Plummer et al., Silicon VLSI Technology, Prentice Hall; 2 edition (Nov.11, 2008); and James D. Plummer et al., Silicon VLSI Technology:Fundamentals, Practice, and Modeling, Pearson; 1 edition (Jul. 24,2000), both of which are hereby incorporated by reference herein intheir entireties. It is emphasized that while some individual processingsteps may be set forth herein, those steps are merely illustrative, andone skilled in the art may be familiar with several equally suitablealternatives that would also fall within the scope of the invention.

It is to be appreciated that the various layers and/or regions shown inthe accompanying figures may not be drawn to scale. Furthermore, one ormore semiconductor layers of a type commonly used in such integratedcircuit devices may not be explicitly shown in a given figure for easeof explanation. This does not imply that the semiconductor layer(s) notexplicitly shown are omitted in the actual integrated circuit device.

FIGS. 1A and 1B depict at least a portion of an exemplary terahertz(THz) detector structure 100 based on a p-n junction diode fabricatedusing a complementary metal-oxide-semiconductor (CMOS) process, and adetector circuit 150 employing the p-n junction diode, respectively.Specifically, FIG. 1A depicts top plan (upper) and cross-sectional(lower) views of the detector structure 100. With reference to FIG. 1A,the detector structure 100 includes a substrate 102, which in thisembodiment is a p-type substrate (p-sub), and a buried well 104, whichin this embodiment is an n-type well (n-well), formed in the substrate.Shallow trench isolation (STI) regions 106 are formed at least partiallythrough the substrate 102 proximate an upper surface of the substrate.

A first implant layer 108, which in this embodiment is an n++ implantlayer, is formed on the n-type well 104 between adjacent STI regions 106proximate the upper surface of the substrate 102. The first implantlayer 108 may be formed, for example, using ion implantation of aprescribed dopant (e.g., phosphorous) and doping concentration level,followed by diffusion, as will be known by those skilled in the art.This first implant layer 108 is electrically connected with the n-typewell 104 and forms a cathode of the p-n junction diode. A second implantlayer 110, which in this embodiment is a p++ implant layer, is formedproximate the upper surface of the substrate between laterally adjacentn++ implant regions 108 and is electrically isolated from the n++implant regions by STI regions as shown. The second implant layer 110may be formed, for example, using ion implantation of a known dopant(e.g., boron) and doping concentration level, in a manner consistentwith the formation of the first implant layer 108. A depletion layer 112is formed beneath the second implant layer 110. This second implantlayer 110 forms an anode of the p-n junction diode.

An interlayer dielectric (ILD) layer 114 is formed over the uppersurface of the structure 100. A plurality of contacts 116 are formedthrough the ILD layer 114. A first subset of the contacts 116 iselectrically connected with the n++ implant layer 108 and form cathodecontacts of the device, while a second subset of the contacts iselectrically connected with the p++ implant layer 110 and forms theanode contact of the device. A first metal layer (M1) is formed on theILD layer 114 which serves as interconnect wiring for the device.

FIG. 1B is a circuit diagram depicting at least a portion of anexemplary detector circuit 150 which uses a p-n junction diode 152. Thep-n junction diode 152 may, in one or more embodiments, be fabricated ina manner consistent with the illustrative detector structure 100 shownin FIG. 1A. The detector circuit 150 include a patch antenna 154 coupledwith an anode of the diode 152, and a cathode of the diode is coupled toground. A first terminal of a bias resistor, R_(B), is connected to anode of the patch antenna, which forms a virtual radio frequency (RF)ground, and a second terminal of the bias resistor is connected to abias voltage source, V_(B). Using the virtual RF ground induced in thepatch antenna directly for DC bias is more efficient for frequenciesgreater than 1 THz. With this biasing arrangement, a direct current (DC)bias current, I_(B), equal to V_(B)/R_(B), will be supplied to the diode152 through the patch antenna 154. The patch antenna 154 is adapted toreceive a transmitted signal supplied thereto and to generate an outputsignal, V_(SIG), at an output of the detector circuit 150.

In FIG. 1A, certain device dimensions are labeled. For example, L₁represents hole diffusion path length, L₂ represents a thickness of theSTI region surrounding the anode of the p-n junction diode (i.e., athickness of the STI region separating the n++ cathode from the p++anode implant layers), L₃ represents cathode contact separation, andL_(S) represents anode width. In this exemplary embodiment, L₁=0.4-0.5μm, L₂=0.16 μm, L₃=0.89 μm, and L_(S)=0.2 μm. These dimensions, alongwith n-well sheet resistance, R_(sh,nw), (e.g., 585 ohms/square in thisembodiment), will affect one or more characterization parameters of thedevice. For example, hole diffusion path length L₁ and n-well sheetresistance R_(sh,nw) are critical design parameters influencing theoperational frequency of the p-n junction diode. Thus, although theperformance of THz detectors based on p-n junction diodes may beadvantageous due to lower noise, the detection frequency is limited bydiffusion path length and series (sheet) resistance of the n-well.

In order to achieve a detector having superior high-frequencyperformance while maintaining the superior low-noise performance of ap-n junction diode, aspects of the present invention provide a noveldiode structure that substantially reduces an effective path length forcarrier diffusion, and substantially reduces an effective seriesresistance. FIGS. 2-10 are cross-sectional views depicting variousillustrative process stages in the fabrication of an exemplary THzdetector structure 200, according to an embodiment of the invention.

With reference now to FIG. 2, the detector structure 200 starts with asilicon-on-insulator (SOI) substrate comprising a carrier substrate 202,which in this embodiment is formed of silicon (e.g., silicon wafer), anoxide or other insulating layer 204 formed on at least a portion of anupper surface of the carrier substrate, and an active device layer 206,which in this embodiment is a silicon (SOI) layer, formed on at least aportion of an upper surface of the oxide layer. The active device layermay have a prescribed conductivity type (e.g., n-type or p-type)associated therewith, such as by using a standard doping process. Inthis illustrative embodiment, the active device layer 206 is of n-typeconductivity (e.g., using a phosphorous dopant). Since the oxide layer204 is formed between the two silicon layers 202, 206 it is oftenreferred to as a buried oxide (BOX) layer. The SOI layer 206, buriedoxide layer 204 and carrier substrate 202 are considered elements of theSOI substrate.

As shown in FIG. 3, circuit components 302, which may include, forexample, transistors, peripheral devices, circuits, sub-circuits,antenna (e.g., patch antenna), etc., are fabricated in the SOI layer 206using techniques known in the art (e.g., standard photolithographicprocessing). A p-n junction diode of the detector is also formed in theSOI layer 206. The p-n junction diode comprises a first doped region304, formed on an upper surface of the SOI layer 206. The first dopedregion 304, in this embodiment, is a heavily-doped p⁺ region, which maybe formed using known techniques, such as, for example, ion implantationof a p-type impurity (e.g., boron) of a prescribed concentration level.Implantation is then followed by rapid thermal annealing (RTA) or pulsedlaser annealing, or an alternative diffusion process, to form a shallowp+/n junction with the underlying n-doped SOI layer 206. The p+ region304 will form an anode of the p-n junction diode and the n-doped SOIlayer 206 underlying the p+ region will form a base of the diode.

Region 306 represents a depletion region of the p-n junction diode whichforms at an interface between the p+ region 304 and underlying n-dopedSOI layer 206. The depletion region 306 defines an area of the n-dopedSOI layer 206 which has been depleted of free charge carriers, caused atleast in part by its proximity to the p⁺ region 304. A depth of thedepletion region 306 can be increased or decreased as a function of thepolarity and/or magnitude of an applied bias (positive bias on an anodewith respect to a cathode of the p-n junction diode decreases the width,whereas negative bias increases the width). The depleted portion of thep+ region 304 is negligible due to its high doping concentration, asknown in the art, and therefore not illustrated for simplicity.

Alternatively, the p⁺/n junction may be formed by epitaxial growth; forexample epitaxial growth of p⁺ hydrogenated crystalline silicon (c-Si:H)with 5-40 atomic % hydrogen using, for example, plasma-enhanced chemicalvapor deposition (PECVD), at temperatures below 400° C., and morepreferably below 200° C., to form an abrupt shallow junction. Thehydrogen content in the hydrogenated crystalline silicon may or may notbe uniform. For example, in one or more embodiments, the hydrogencontent has a gradient towards the c-Si:H/Si interface. In one or moreembodiments, the PECVD silicon may additionally contain one or moreelements, such as, but not limited to, deuterium (D), fluorine (F),chlorine (Cl), carbon (C), germanium (Ge), oxygen (O) and nitrogen (N).Furthermore, in one or more embodiments, hot-wire chemical vapordeposition (HWCVD) may be used instead of PECVD.

The hydrogenated crystalline silicon (c-Si:H) containing layer may begrown from a mixture of a precursor gas, such as silane (SiH₄) or othergases of the Si_(x)H_(y) family, such as disilane (althoughfluorine-containing and/or chlorine-containing precursors may be used aswell (e.g., SiF₄ or H₂SiCl₂), and combinations thereof), a carrier gas(e.g., H₂) and a dopant gas, such that [H₂]/[SiH₄]>5 in a PECVD reactor.For n-type doping, the dopant gas may include, for example, phosphine(PH₃). For p-type doping, the dopant gas may include diborane (B₂H₆) ortrimethylborane, B(CH₃)₃.

A heavily-doped n-type region 308 (n++) is preferably formed on aportion of the n-type SOI layer 206, spaced laterally and electricallyisolated from the p+ region 304. The n++ region 308 and underlying SOIlayer 206 is one convenient method of connecting a top metalinterconnect (e.g., 314) to a bottom electrode metal (e.g., 1002 in FIG.10) so that the cathode of the p-n junction diode can be accessed froman upper surface of the substrate. In other embodiments, the bottomelectrode metal can be connected directly to the top metal interconnect,for example through an opening formed in a shallow trench isolation(STI) structure, as will be described in further detail in conjunctionwith FIGS. 11-18.

In order to electrically isolate active devices formed in the SOIsubstrate, a plurality of STI structures 310 are preferably formed inthe SOI layer 206 proximate the upper surface of the substrate andextending vertically through the SOI layer to the buried oxide layer204. An interlayer dielectric (ILD) layer 312, or alternative insulatinglayer, is preferably formed over an upper surface of the SOI substrate206 as shown. The ILD layer is comprised of dielectric material used toelectrically separate closely spaced interconnect lines, such asinterconnect and/or contact metallization 314, typically arranged inseveral levels (in a multilevel metallization process) in advancedintegrated circuits; ILD generally features a low dielectric constant(low-k), for example as close to 1 as possible, to minimize capacitivecoupling (e.g., crosstalk) between adjacent metal lines. The contactmetallization 314 provides electrical connection to the n++ region 308,forming a top cathode contact of the p-n junction diode as previouslystated, and the p+ region 304, forming the anode contact of the p-njunction diode, as well as providing electrical connection to one ormore other circuit components 302 formed in the SOI substrate. It willbe appreciated that in other embodiments, multiple ILD and metallizationlevels may be used as is customary in the fabrication of integratedcircuits, and as such the electrical connections to the n++ region 308,p+ region 304 and one or more circuit components 302 may be obtainedusing contact metallization from different metallization levels.

With reference now to FIG. 4, the SOI substrate is bonded to either atemporary handle substrate 402, which is subsequently removed beforepackaging, or a permanent packaging, which may comprise through-siliconor through-glass vias, silicon interposers, etc., as known in the art.The handle substrate 402, in one or more embodiments, comprises siliconor glass, although embodiments of the invention are limited to suchmaterials. In some embodiments, the handle substrate 402 comprises amechanically flexible material such as plastic or dielectric-coatedmetal foil.

As shown in FIG. 5, the carrier substrate (202 in FIG. 4) is removedusing techniques known in the art, such as, for example, mechanicalgrinding or controlled spalling technology (CST)—in the latter case, thehandle substrate is chosen to be flexible and may also contain astressor layer, as known in the art, followed by chemical etching. Inone or more embodiments, the etchant used in the chemical etching isselective to silicon (or other material used to form the carriersubstrate), and the buried oxide layer 204 is used as an etch-stop layerso that etching halts when the buried oxide layer is reached.Tetra-methyl-ammonium-hydroxide (TMAH) or potassium hydroxide (KOH)solution is used as an etchant in one or more embodiments, although theinvention is not limited to any specific etchant. Reactive-ion-etching(ME) using, for example, SF₆/O₂, SF₆/CHF₃, SF₆/CCl₂F₂ or CF₄, may alsobe used in some embodiments.

In FIG. 6, once the carrier substrate (202 in FIG. 4) has been removed,thereby exposing the buried oxide layer 204, a first via 602 is openedthrough the buried oxide layer to access a backside of the detector (p-njunction diode cathode), using fabrication techniques known in the art,such as, for example, standard lithography and etching. Specifically,the via 602 is formed through the buried oxide layer 204 to expose abackside surface of at least a portion of the n-doped SOI layer 206 onwhich the p+ region 304 and depletion region 306 are formed.

A back contact (cathode) 702 of the detector (p-n junction diode) isformed on the backside of the SOI layer 206 within via 602, as shown inFIG. 7. More particularly, in one or more embodiments, the back contact702 of the detector is formed by epitaxial growth of n⁺-dopedhydrogenated crystalline silicon (n⁺ c-Si:H) with 5-40 atomic % hydrogenusing e.g. PECVD, at temperatures below 400° C., and more preferablybelow 200° C., in a manner consistent with the formation of the n++region 308 shown in FIG. 3 and previously described. This back contact702 forms a bottom cathode contact region of the p-n junction diode, inthis illustrative embodiment. The amorphous portion of hydrogenatedsilicon (a-Si:H) grown on the exposed backside of the STI structures 310through via 602 may be removed using selective etching, leaving behindthe crystalline portion (as illustrated) for example using H₂ plasmawhich may be applied in-situ. Alternatively (not explicitly shown, butimplied), the grown n⁺-doped hydrogenated Si (n⁺ Si:H) layer may bepatterned either using an additional mask (which will leave n⁺ a-Si:H onat least a portion of the bottom surface of the exposed STI) or usingbottom contact metal of a subsequent process step (described below) as ahard mask (which will leave n⁺ a-Si:H on the entire exposed bottomsurface of the STI, sidewalls of BOX in via 602, and on a portion of thebackside surface of BOX covered by the bottom metal contact 1002).

In embodiments where selective etching is performed, hydrogenatedamorphous silicon may be etched selectively with respect to hydrogenatedcrystalline silicon, with selectively in a range of about 3:1 to 10:1;in one or more other embodiments, the selectivity is greater than about10:1. Furthermore, in embodiments where selective etching is performed,the selective etching may be performed in hydrogen plasma, for examplein-situ; that is, in the same reactor where hydrogenated silicon (Si:H)has been grown. In accordance with one or more other embodiments, a wetetch, such as, for example, dilute KOH (˜1M or below) or dilute HF (<5%is DI water), is used. In some embodiments the hydrogenated crystallinesilicon layer may contain germanium (Ge); that is, comprised ofc-SiGe:H, wherein Ge is introduced from a gas mixture containing aprecursor such as GeH4, wherein [H₂]/([SiH₄]+[GeH₄])>5.

As previously stated, the n++ region 308 provides access to the backcontact (cathode) 702. With reference now to FIG. 8, a second via 802 isopened through the buried oxide layer 204 (and n+a-Si:H when present onBOX, e.g. in embodiments where bottom contact metal 1002 is used as ahard mask for patterning n+Si:H instead of selective etching, asdescribed earlier) to expose a backside of at least a portion of the SOIlayer 206 directly beneath and proximate to the highly-doped n++ region304. In this manner, the via 802 and n++ region 308 provide access to anarea designated to electrically connect to a back electrode (a cathodeback contact, in this embodiment) of the detector. To accomplish this,the via 802 is further etched through the exposed SOI layer 206 toaccess the highly-doped n⁺⁺ region 308, as shown in FIG. 9. This may beaccomplished, in one or more embodiments, using a selective wet-etch,such as, for example, tetramethylammonium hydroxide (TMAH) or KOH, ortimed etching, such as, for example, reactive ion etching (ME).

FIG. 10 illustrates a metallization step, wherein metal is deposited inthe first and second vias 602 and 802, respectively, to form a cathodeback electrode 1002 of the detector. More particularly, the cathode backelectrode 1002 is electrically connected with the n⁺⁺ region 308 and theback contact (cathode) 702 of the detector. In embodiments where theback electrode 1002 is used as a hard mask for patterning Si:H insteadof selective etching, the portion of n⁺ a-Si:H on the back surface ofBOX not covered by the back electrode 1002 may be etched at this stage.In one example, RIE e.g. with SF₆/CHF₃, CCl₂F₂/O₂ or CF₄ may be used forthis purpose.

Optionally, a low-temperature annealing step may be used to improveohmic contact and/or silicide formation. In a silicide process, silicidecontacts are formed only in those areas in which deposited metal (whichafter annealing becomes a metal component of the silicide) is in directcontact with silicon (e.g., n++ region 308 and back contact 702, both ofwhich are formed of doped silicon), hence, the process is self-aligned.The temperature and/or duration of the anneal step is preferably limitedso as to prevent substantial out-diffusion of hydrogen from the c-Si:Hlayer(s).

If a temporary handle substrate 402 has been used, it may be removed atthis stage after attaching the structure to a second (temporary orpermanent) handle substrate on an opposite side of the buried oxidelayer 204. Packaging using standard techniques may follow, as willbecome apparent to those skilled in the art.

Rather than forming a conductive path between the top and bottomsurfaces of the SOI layer using the heavily-doped n++ region 308 andunderlying portion of the n+ SOI layer 206, as shown in FIG. 10, thedetector structure can utilize a direct connection between the cathodeback electrode 1002 and the metal interconnect layer (contactmetallization) 314, according to an alternative embodiment.Specifically, FIGS. 11-18 are cross-sectional views depicting variousstages of an illustrative semiconductor fabrication process in theformation of an exemplary THz detector structure 1100, according toanother embodiment of the invention.

As shown in FIG. 11, the detector structure 1100 is fabricated on an SOIsubstrate comprising a carrier substrate 202, a buried oxide (BOX) orother insulating layer 204 formed on an upper surface of the carriersubstrate, and an active silicon device (SOI) layer 206 formed on anupper surface of the buried oxide layer 204. Circuit components 302,which may include, but are not limited to, transistors, peripheraldevices, circuits, sub-circuits, antenna (e.g., patch antenna), etc.,are fabricated in the SOI layer 206, as previously stated. A p-njunction diode of the detector is also formed in the SOI layer 206. Thep-n junction diode includes the first doped region 304, which, in thisembodiment, is a heavily-doped p⁺ region, formed on the upper surface ofthe n-doped SOI layer 206 using known techniques. The p+ region 304 willform an anode of the p-n junction diode and the n-doped SOI layer 206underlying the p+ region will form a base of the diode.

As previously described in connection with FIG. 3, the p-n junction ofthe detector is formed using standard integrated circuit fabricationtechniques, such as ion implantation followed by rapid thermal annealingor pulsed laser annealing to form a shallow junction. Alternatively, thep-n junction may be formed by epitaxial growth of p+c-Si:H with 5-40atomic % hydrogen using, for example, PECVD at temperatures below 400°C., and more preferably below 200° C., to form an abrupt shallowjunction.

A plurality of STI structures 310 are preferably formed in the SOI layer206 proximate the upper surface of the substrate and extendingvertically through the SOI layer to the buried oxide layer 204. Aninterlayer dielectric (ILD) layer 312, or alternative insulating layer,is preferably formed over an upper surface of the SOI substrate 206 asshown and is used to electrically separate closely spaced interconnectlines, such as interconnect and/or contact metallization 314. Thecontact metallization 314 provides electrical connection to the circuitcomponents 302, as well as the p-n junction diode, formed in the SOIsubstrate. As previously described with respect to FIG. 3, multiple ILD312 and metallization 314 levels may be used in some embodiments.

As depicted in FIG. 12, the substrate is preferably bonded to either atemporary handle substrate 402 which is subsequently removed prior topackaging, or a permanent packaging comprised of through vias, as willbe known in the art. The carrier substrate 202 is then removed usingtechniques known in the art, such as, e.g., mechanical grinding orcontrolled spalling (in the latter case, the handle substrate is chosento be flexible and may also contain a stressor layer), followed bychemical etching of the silicon, selectively stopping on the buriedoxide layer 204. The resulting structure is shown in FIG. 13.

With reference to FIG. 14, a via 1402 is opened in the buried oxidelayer 204, through a back surface thereof, to access a backside of then-doped SOI layer 206 underlying the depletion region 306. The via 1402is formed, in one or more embodiments, using known techniques, such as,for example, standard lithography and etching. A back contact 1502 ofthe detector is formed in the via 1402, as shown in FIG. 15. In one ormore embodiments, the via 1402 is wider than the back contact 1502, sothat conductive material deposited in the via during a subsequentprocessing step surrounds sidewalls of the back contact 1502, therebyreducing an electrical resistance between the back cathode electrode(see FIG. 18) and the back contact 1502. The back contact 1502 provideselectrical connection with the n-doped SOI layer (base region) 206underlying the depletion region 306 and forms the cathode of the p-njunction diode.

In one or more embodiments, the back contact 1502 is formed by epitaxialgrowth of n+c-Si:H with 5-40 atomic % hydrogen using, for example, PECVDat temperatures below 400° C., and more preferably below 200° C. Like inthe formation of the back contact 702 shown in FIG. 7, the amorphousportion of Si:H grown on the back surface of the STI structures 310exposed through the via 1402 may be removed in-situ, for example usingH2 plasma (as illustrated in FIG. 15). Alternatively, n+ Si:H may besubsequently patterned, for example using a separate lithography mask orusing the bottom contact metal of a succeeding metal deposition step asa hard mask.

In FIG. 16, a second via 1602 is opened through the buried oxide layer204 to expose a portion of the STI structure 310 directly underneath acorresponding portion of the metal interconnect 314 forming a topcathode contact of the p-n junction diode. The via 1602 is furtheretched through the STI structure 310, as shown in FIG. 17, to expose theunderside of the upper metal interconnect 314 ultimately forming the topcathode contact of the p-n junction diode. This may be accomplished, forexample, by selective RIE (e.g., a CF₄/O₂ plasma), or an alternativeetching process.

FIG. 18 illustrates a metallization process used to form a back cathodeelectrode 1802 of the detector. The metal deposited in the via 1602forms a conductive structure (e.g., pillar) which electrically connectsthe back cathode electrode 1802 of the detector with the overlying topmetal interconnect 314 to form the top cathode contact of the detector.In this manner, the embodiment shown in FIG. 18 eliminates the need toform a heavily-doped n++ region 308 for providing electrical connectionbetween the top and back surfaces of the active device layer 206.

Optionally, in one or embodiments, a low-temperature annealing step maybe used to improve ohmic contact and/or silicide formation, aspreviously described in connection with FIG. 10. The temperature and/orduration of the anneal step is limited to prevent substantialout-diffusion of hydrogen from the c-Si:H layer(s). In one example, 50%of the hydrogen content of c-Si:H is lost during an annealing time ofapproximately 25 hours at an annealing temperature of 350° C., thereforethe annealing time is limited to 2-3 hours. In another example, 50% ofthe hydrogen content of c-Si:H is lost during an annealing time ofapproximately 30 minutes at an annealing temperature of 500° C.,therefore the annealing time is limited to 30 minutes. In yet anotherexample, 50% of the hydrogen content of the hydrogenated siliconcontaining material is lost during an annealing time of approximately 5minutes at an annealing temperature of 600° C.

If a temporary handle substrate has been used, it may be removed at thisstage after attaching the structure to a secondary (temporary orpermanent) handle substrate on the opposite (BOX) side. Packaging usingstandard techniques known in the art may then follow.

Consider the exemplary detector circuit 150 employing a p-n junctiondiode 152 shown in FIG. 1B. The output signal V_(SIG) generated by thedetector circuit 150 may be supplied to an input of a low-noiseamplifier (narrow band), or other sensitive circuitry coupled with thedetector circuit. In this illustration, an incident signal is receivedby the patch antenna 154. Furthermore, consider an incident signal 1900having a waveform as depicted in FIG. 19. The incident signal 1900includes an information signal 1902 and a carrier signal 1904, with theinformation signal having a frequency that is substantially lower thanthe carrier frequency. By way of example only and without limitation,assume the information signal 1902 is in an RF frequency range and thecarrier signal is in a THz frequency range.

Several parameters of the diode 152 used in the detector circuit 150will affect the high-frequency operation of the circuit. Such parametersinclude, among other factors, junction capacitance, C_(j), diffusioncapacitance, C_(diff), depletion capacitance, C_(dep), diode current,I_(D), and hole transit time, τ_(p). A relationship between theseparameters and frequency can be expressed as follows:

$\quad\left\{ \begin{matrix}{C_{j} = {C_{dep} + C_{diff}}} \\{C_{diff} = {\frac{{qI}_{D}}{kT}\left( {{\tau_{p}/2}\omega} \right)}}\end{matrix} \right.$where k is Boltzman's constant, T is junction temperature (in degreesKelvin), and the quantity kT/q is referred to as the thermal voltage ofthe p-n junction (approximately 26 mV at room temperature).

For high-frequency operation of the detector, it is desirable to reducethe diffusion capacitance C_(diff): However, the diode current I_(D) ispreferably sufficiently large (e.g., at least 10 times higher than thebackground noise current, in some preferred embodiments). If I_(D) isreduced in order to reduce the diffusion capacitance, the noise fed intothe low-noise amplifier reduces the signal-to-noise ratio or maycompletely obscure the signal. Thus, an important advantage ofembodiments of the invention is that diffusion capacitance issignificantly reduced by the novel p-n junction diode structure (e.g.,FIG. 10), and such reduction is independent of diode current.

By way of illustration only, consider the following expression:τ_(p)=L_(p,eff) ²/D_(p), where L_(p,eff) is the effective path lengthfor hole diffusion and D_(p) is the diffusion coefficient for holes.Conventionally, L_(p,eff) is approximately 0.4-0.5 μm, given thatτ_(p)=˜100 ps (See, e.g., Z. Ahmad, et. al., “THz Detection Usingp+/n-Well Diodes Fabricated in 45-nm CMOS,” IEEE Elect. Dev. Lett. vol.37, no. 7, pp. 823-826, 2016, incorporated by reference herein). Bycomparison, L_(p,eff) for the diode structure formed in accordance withembodiments of the invention is determined by SOI thickness; that is,about 20-50 nm, in one or more embodiments. Therefore, C_(diff) isbeneficially reduced by at least a factor of one hundred times using thenovel diode structure according to embodiments of the invention.

As a secondary advantage, the novel diode structure according toembodiments of the invention is a reduction in series resistance, R_(S),of the device. By way of illustration only, R_(S) is typically about 210ohms (Ω) for a diode area A=0.12 μm′ and N_(D)=2×10¹⁷ cm⁻³. Bycomparison, in accordance with embodiments of the invention, for thesame diode parameters A and N_(D) as above, SOI thickness less thanabout 50 nm will be fully depleted (W_(D)=˜50 nm at V_(D)=0.7V),therefore R_(S), will depend on the contact resistance. If specificcontact resistance is less than about 2×10⁻⁷ Ω·cm², which is reasonableto achieve, then R_(S)<210Ω can be attained. Using the diode structureaccording to embodiments of the invention, for the same diode area A asabove and N_(D)=2×10¹⁸ cm⁻³, W_(D)=˜15 nm at V_(D)=0.7V, therefore for50 nm SOI, calculated R_(s)≈16Ω. In order for the contact resistance tonot dominate, specific contact resistance should be less than about2×10⁻⁸ Ω·cm², which is still attainable.

FIG. 20 is a schematic diagram depicting at least a portion of anexemplary detection circuit 2000 in which aspects of the presentinvention may be employed, according to an embodiment of the invention.The detection circuit 2000 utilizes a pixel array 2002 comprising aplurality of detectors 2004 arranged in a grid configuration, with eachdetector connected to a unique column and row pair as shown. Each of atleast a subset of the detectors 2004 comprises a detector fabricated inaccordance with one or more embodiments of the invention. Bias circuitry2006 coupled to the pixel array 2002 supplies a bias current, I_(BIAS),to each of the rows of detectors 2004, as a function of at least onereference signal, REF, supplied to the bias circuitry.

A column address decoder 2008 is included in the circuit 2000 which isconnected to the pixel array 2002 and is configured to select a givenone or more of the columns in the pixel array as a function of at leastone column control signal supplied to the column address decoder. Eachof the rows of the pixel array 2002 are connected to a first multiplexer(MUX) 2010, which in this embodiment is an analog multiplexer. The firstmultiplexer 2010 is adapted to receive at least one row control signalfor selecting a given one or more of the rows in the pixel array 2002.Thus, a unique combination of column control signal and row controlsignal is used to select a given one of the detectors 2004 in the pixelarray 2002, and an output signal, V_(SIG), generated by a selecteddetector 2004 is then fed to an input of a corresponding amplifier in anamplifier bank 2012.

An output of each of a plurality of amplifiers in the amplifier bank2012 is fed to a second multiplexer 2014, which in this embodiment is ananalog multiplexer. The second multiplexer 2014 is configured togenerate either a serial output, shown in dotted line, or as paralleloutputs, shown in solid lines, as a function of a serial/parallelcontrol signal supplied to the second multiplexer 2014.

At least a portion of the techniques of the present invention may beimplemented in an integrated circuit. In forming integrated circuits,identical die are typically fabricated in a repeated pattern on asurface of a semiconductor wafer. Each die includes a device describedherein, and may include other structures and/or circuits. The individualdie are cut or diced from the wafer, then packaged as an integratedcircuit. One skilled in the art would know how to dice wafers andpackage die to produce integrated circuits. Any of the exemplarycircuits illustrated in the accompanying figures, or portions thereof,may be part of an integrated circuit. Integrated circuits somanufactured are considered part of this invention.

Those skilled in the art will appreciate that the exemplary structuresdiscussed above can be distributed in raw form (i.e., a single waferhaving multiple unpackaged chips), as bare dies, in packaged form, orincorporated as parts of intermediate products or end products thatbenefit from having high-frequency detector devices, and moreparticularly high-frequency p-n junction diodes, formed in accordancewith one or more embodiments of the invention.

The illustrations of embodiments of the invention described herein areintended to provide a general understanding of the various embodiments,and they are not intended to serve as a complete description of all theelements and features of apparatus and systems that might make use ofthe circuits and techniques described herein. Many other embodimentswill become apparent to those skilled in the art given the teachingsherein; other embodiments are utilized and derived therefrom, such thatstructural and logical substitutions and changes can be made withoutdeparting from the scope of this disclosure. The drawings are alsomerely representational and are not drawn to scale. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

Embodiments of the invention are referred to herein, individually and/orcollectively, by the term “embodiment” merely for convenience andwithout intending to limit the scope of this application to any singleembodiment or inventive concept if more than one is, in fact, shown.Thus, although specific embodiments have been illustrated and describedherein, it should be understood that an arrangement achieving the samepurpose can be substituted for the specific embodiment(s) shown; thatis, this disclosure is intended to cover any and all adaptations orvariations of various embodiments. Combinations of the aboveembodiments, and other embodiments not specifically described herein,will become apparent to those of skill in the art given the teachingsherein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groupsthereof. Relational terms such as “upper,” “above,” “backside” and“below” are used to indicate relative positioning of elements orstructures to each other as opposed to absolute elevation or position.

The corresponding structures, materials, acts, and equivalents of allmeans or step-plus-function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the various embodiments has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the forms disclosed. Many modifications and variations willbe apparent to those of ordinary skill in the art without departing fromthe scope and spirit of the invention. The embodiments were chosen anddescribed in order to best explain the principles of the invention andthe practical application, and to enable others of ordinary skill in theart to understand the various embodiments with various modifications asare suited to the particular use contemplated.

The abstract is provided to comply with 37 C.F.R. § 1.72(b), whichrequires an abstract that will allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the appended claims reflect,inventive subject matter lies in less than all features of a singleembodiment. Thus the following claims are hereby incorporated into theDetailed Description, with each claim standing on its own as separatelyclaimed subject matter.

Given the teachings of embodiments of the invention provided herein, oneof ordinary skill in the art will be able to contemplate otherimplementations and applications of the techniques of embodiments of theinvention. Although illustrative embodiments of the invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that embodiments of the invention are not limited to thoseprecise embodiments, and that various other changes and modificationsare made therein by one skilled in the art without departing from thescope of the appended claims.

What is claimed is:
 1. A method of forming a semiconductor detectorstructure, the method comprising: forming a p-n junction diode in anactive device layer of a silicon-on-insulator (SOI) substrate, theactive device layer being formed on an upper surface of an insulatorlayer of the SOI substrate, the insulator layer being formed on an uppersurface of a carrier substrate of the SOI substrate; removing thecarrier substrate to thereby expose the insulator layer; forming a firstopening through the insulator layer to access a backside of a firstdoped region of the p-n junction diode, the first doped regionunderlying a second doped region of the p-n junction diode, the firstdoped region having a first conductivity type associated therewith andthe second doped region having a second conductivity type associatedtherewith which is opposite in polarity to the first conductivity type;forming a back contact on a back surface of the first doped region andelectrically connecting with the first doped region, the back contactbeing formed by epitaxially growing doped hydrogenated crystallinesilicon in the first opening; forming a conductive interconnect layer onan upper surface of the SOI substrate, the interconnect layer comprisinga first top contact providing electrical connection with the seconddoped region; and forming an electrode of the p-n junction diode in thefirst opening on the backside of the detector structure, the electrodeproviding electrical connection with the back contact of the p-njunction diode.
 2. The method of claim 1, wherein the back contact isformed by epitaxially growing n-doped hydrogenated crystalline siliconwith 5 to 40 atomic percent hydrogen using plasma-enhanced chemicalvapor deposition at temperatures below 400 degrees Celsius.
 3. Themethod of claim 1, wherein the second doped region is formed byimplanting a p-type impurity of a prescribed concentration levelfollowed by annealing.
 4. The method of claim 1, wherein the seconddoped region is formed on the first doped region by epitaxially growingp⁺-doped hydrogenated crystalline silicon with 5 to 40 atomic percenthydrogen using plasma-enhanced chemical vapor deposition at temperaturesbelow 400 degrees Celsius.
 5. The method of claim 1, further comprisingforming shallow trench isolation (STI) structures in the active devicelayer proximate an upper surface of the SOI substrate and extendingvertically through the active device layer to the insulator layer, thefirst and second doped regions being formed between two laterallyadjacent STI structures.
 6. The method of claim 5, wherein the firstopening is formed between adjacent STI structures, the first openingexposing a back surface of each of the adjacent STI structures, themethod further comprising: epitaxially growing n-doped hydrogenatedsilicon in the first opening; and removing an amorphous portion of thehydrogenated silicon grown on the exposed back surfaces of the STIstructures using selective etching, leaving behind an n⁺-dopedhydrogenated crystalline silicon portion forming the back contact of thep-n junction diode.
 7. The method of claim 1, further comprising forminga dielectric layer on at least a portion of an upper surface of theactive device layer of the SOI substrate.
 8. The method of claim 7,further comprising bonding a handle substrate to an upper surface of thedielectric layer.
 9. The method of claim 7, wherein the dielectric layercomprises an interlayer dielectric (ILD) layer.
 10. A method of forminga semiconductor detector structure, the method comprising: forming a p-njunction diode in an active device layer of a silicon-on-insulator (SOI)substrate, the active device layer being formed on an upper surface ofan insulator layer of the SOI substrate, the insulator layer beingformed on an upper surface of a carrier substrate of the SOI substrate;removing the carrier substrate to thereby expose the insulator layer;forming a first opening through the insulator layer to access a backsideof a first doped region of the p-n junction diode, the first dopedregion underlying a second doped region of the p-n junction diode, thefirst doped region having a first conductivity type associated therewithand the second doped region having a second conductivity type associatedtherewith which is opposite in polarity to the first conductivity type;forming a back contact on a back surface of the first doped region andelectrically connecting with the first doped region; forming aconductive interconnect layer on an upper surface of the SOI substrate,the interconnect layer comprising a first top contact providingelectrical connection with the second doped region; forming an electrodeof the p-n junction diode in the first opening on the backside of thedetector structure, the electrode providing electrical connection withthe back contact of the p-n junction diode; forming shallow trenchisolation (STI) structures in the active device layer proximate an uppersurface of the SOI substrate and extending vertically through the activedevice layer to the insulator layer, the first and second doped regionsbeing formed between two laterally adjacent STI structures; and forminga conductive structure through at least a given one of the STIstructures, the conductive structure providing electrical connectionbetween a top contact formed on an upper surface of the SOI substrateand the electrode formed on the backside of the detector structure. 11.The method of claim 10, wherein forming the conductive structurecomprises: forming a second opening through the insulator layer andthrough the given one of the STI structures to expose a back surface ofthe top contact; and depositing conductive material in the first andsecond openings to form the electrode.
 12. A method of forming asemiconductor detector structure, the method comprising: forming a p-njunction diode in an active device layer of a silicon-on-insulator (SOI)substrate, the active device layer being formed on an upper surface ofan insulator layer of the SOI substrate, the insulator layer beingformed on an upper surface of a carrier substrate of the SOI substrate;removing the carrier substrate to thereby expose the insulator layer;forming a first opening through the insulator layer to access a backsideof a first doped region of the p-n junction diode, the first dopedregion underlying a second doped region of the p-n junction diode, thefirst doped region having a first conductivity type associated therewithand the second doped region having a second conductivity type associatedtherewith which is opposite in polarity to the first conductivity type;forming a back contact on a back surface of the first doped region andelectrically connecting with the first doped region; forming aconductive interconnect layer on an upper surface of the SOI substrate,the interconnect layer comprising a first top contact providingelectrical connection with the second doped region; forming an electrodeof the p-n junction diode in the first opening on the backside of thedetector structure, the electrode providing electrical connection withthe back contact of the p-n junction diode; and forming a conductivestructure through the SOI substrate, the conductive structure beingelectrically isolated from the second doped region and, in conjunctionwith the electrode, providing electrical connection between a second topcontact formed on the upper surface of the SOI substrate and the firstdoped region.